1. Field of the Invention
The present invention relates to piezoelectric oscillators, and more particularly, to a temperature-compensated piezoelectric oscillator that compensates for an oscillation frequency in accordance with an ambient temperature and also relates to an electronic apparatus including the temperature-compensated piezoelectric oscillator.
2. Description of the Related Art
In general, piezoelectric oscillators include a piezoelectric element, such as a crystal strip, that resonates at a predetermined frequency in accordance with an applied voltage and an amplifying circuit for amplifying a resonant signal by the piezoelectric element and for outputting the amplified resonant signal. The resonant frequency of the piezoelectric element, such as a crystal strip, is dependent upon the temperature. Thus, even if the same voltage is applied, the resonant frequency is changed as the temperature of the element changes.
In order to solve this problem, a plurality of temperature-compensated piezoelectric oscillators including a variable capacitance element, such as a varactor diode, that is connected to a piezoelectric element and a temperature compensation voltage generation circuit for changing a voltage applied to the variable capacitance element in accordance with the ambient temperature are known (for example, see Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-135053; Patent Document 2: Japanese Unexamined Patent Application Publication No. 2002-76773; and Patent Document 3: Japanese Unexamined Patent Application Publication No. 6-224635).
In such temperature-compensated piezoelectric oscillators, a resonant frequency depends on a combined capacitance of a piezoelectric element and a variable capacitance element. Adjusting a voltage applied to the variable capacitance element changes the capacitance of the variable capacitance element. As a result, the combined capacitance is changed, and the resonant frequency is changed. By setting the amount of change in the resonant frequency to compensate for the amount of change in the resonant frequency caused by the temperature of the piezoelectric element, a temperature-compensated piezoelectric oscillator that outputs a high-frequency signal having a constant resonant frequency without being affected by the ambient temperature is provided.
In each of the known temperature-compensated piezoelectric oscillators, an output voltage from a temperature compensation voltage generation circuit is applied to one end of a variable capacitance element (for example, a varactor diode), and the other end of the variable capacitance element is grounded or set to a constant voltage.
Such a temperature-compensated piezoelectric oscillator is installed in a mobile communication apparatus or other suitable apparatus, and is used as a reference signal source. In recent years, a reduction in voltage has been required for mobile communication apparatuses. In accordance with this reduction, a reduction in voltage has also been required for temperature-compensated piezoelectric oscillators, which are used as reference signal sources.
Known temperature compensation voltage generation circuits include a thermistor, which is a thermo-sensitive element, as described in the above-mentioned patent documents. Applying a low voltage to the circuit generates an output voltage corresponding to the temperature, and the voltage is supplied to a variable capacitance element. Normally, due to simplification of the circuit, a power supply voltage of the temperature-compensated piezoelectric oscillator is used as a voltage source for supplying the low voltage to the temperature compensation voltage generation circuit.
Thus, as described above, in accordance with the reduction in the voltage in the temperature-compensated piezoelectric oscillator, the voltage supplied to the temperature compensation voltage generation circuit is reduced. As a result, an output voltage, that is, the maximum value of the voltage supplied to the variable capacitance element is reduced. Thus, the range of the voltage applied to the variable capacitance element is reduced, and the range of possible changes in the capacitance is reduced.
In contrast, although the resonant frequency of a piezoelectric element, such as a quartz crystal resonator, is dependent upon a change in the temperature, the resonant frequency does not depend on the applied voltage. Thus, even if the voltage of the temperature-compensated piezoelectric oscillator is reduced, the amount of change in the resonant frequency with respect to a change in the temperature does not change.
Accordingly, a sufficient temperature compensation for the resonant frequency of the piezoelectric element may not be achieved in the range of the voltage generated from the temperature compensation voltage generation circuit.